The present invention is related generally to vehicle wheel service equipment, and in particular, to vehicle wheel service equipment such as a vehicle wheel balancing system, which is configured to measure vehicle wheel assembly configurations, including size, offset, and rear spacing parameters.
A typical vehicle wheel assembly, such as shown in FIG. 1, may be described using a number of conventional measurements and feature descriptions. These measurements include the rim diameter, the rim width, the offset, the backside setting (rear spacing), the caliper clearance, and the bolt circle. Common feature descriptions include the mounting pad, the center bore, the drop center, the rim bell, the safety humps (inner and outer), the bead seats (inner and outer), and the wheel weight contour or flanges (inner and outer). By providing various values for the conventional measurements and features of a vehicle wheel assembly, the general characteristics of the wheel assembly will be known.
Vehicle wheel assemblies, which consist of a tire mounted to a wheel rim, are manufactured and assembled in ever increasing varieties and configurations. Customized vehicles often replace the original equipment vehicle wheel assemblies with wheel assemblies having greater diameters, lower profile tires, and different offset measurements. Some of these changes are made by the vehicle owner for cosmetic purposes, while others are made for purposes of improving vehicle handling or performance characteristics. For example, very large tires mounted to smaller wheel rims are commonly added to off-road vehicles to facilitate low-speed movement over difficult terrain. Conversely, larger wheel rims combined with very low profile tires are often added to performance vehicles such as sports cars, to provide an increase in tire contact patch size and to improve high-speed vehicle performance or vehicle appearance in the eyes of the consumer.
When a vehicle wheel assembly which does not conform to a vehicle manufacturer's original specifications is installed on a vehicle, a number of fitment and vehicle handling problems can arise. For example, by increasing the overall outer diameter of the wheel assembly, either through the installation of a different profile tire on the wheel rim, and/or the use of a larger diameter wheel rim, interference can occur between the wheel assembly and vehicle structure. Similarly, if the wheel rim diameter is decreased, there may not be sufficient clearance for brake components installed on the wheel hub. Once measure which may have a significant effect on the fitment of a vehicle wheel assembly is the offset measurement.
As seen in FIGS. 1 and 2, the offset of a vehicle wheel assembly is the distance between the mounting pad surface and the centerline of the vehicle wheel rim, defined as the midpoint between the inner and outer rim edges. If the mounting pad is axially displaced towards the outside (i.e. street side) of the vehicle wheel assembly, it is referred to as a positive offset. Conversely, if the mounting pad is axially displaced towards the inside (i.e. vehicle centerline), it is referred to as a negative offset. A zero offset indicates the hub mounting surface is at the exact centerline of the rim. An alternative axial measurement associated with the mounting pad is known as the rear or back spacing, and is defined as the distance between the inner rim edge and the mounting pad surface. In the case of a wheel having a zero offset, the rear or back spacing would be equal to ½ the rim width (plus the thickness of the inboard flange on the rim). Offset is usually measured in millimeters (mm) and often has the designation “ET” added to the offset measurement, for example, a 19 mm offset may be listed as ET19. Note that offset and backspacing are related but measured at slightly different points.
Given the structure of most vehicle wheel assemblies, manual measurement of the offset is complicated and often inaccurate, requiring a measurement of the rim width, identification of the wheel rim centerline, and a measurement of the displacement of the mounting pad from the calculated wheel rim centerline.
Accordingly, it would be advantageous to provide a vehicle wheel assembly service system, such as a vehicle wheel balancer system or a vehicle wheel tire changing system, with the functionality necessary to measure the offset or rear spacing of a vehicle wheel assembly, and to display to an operator associated vehicle wheel assembly parameters. It would be further advantageous to provide a vehicle service system with the functionality to utilize measured vehicle wheel assembly offset or rear spacing information, with or without other vehicle wheel assembly parameters, to provide an operator with guidance and/or instruction regarding the installation of the vehicle wheel assembly on a specific type of vehicle. It would be further advantageous to provide the aforementioned functionality in a vehicle wheel assembly service system which is configured to acquire measurements of the physical characteristics of a vehicle wheel assembly, such as during a vehicle wheel balancing or vehicle wheel tire changing procedure.